Display devices comprising liquid crystalline compounds (the term "liquid crystalline compounds" is used in this specification as a general term for the compounds exhibiting a liquid crystal phase and for the compounds which do not exhibit a liquid crystal phase but are useful as a component of liquid crystal compositions) have widely been employed for the display of watches, tabletop calculators, word processors, or the likes. These display devices have employed the optical anisotropy and dielectric anisotropy of liquid crystalline compounds.
While liquid crystal phases include a nematic liquid crystal phase, smectic liquid crystal phase, and cholesteric liquid crystal phase, the nematic liquid crystal phase has most widely been employed. As display mode, dynamic scattering (DS) mode, deformation of aligned phase (DAP) mode, guest/host (GH) mode, twisted nematic (TN) mode, super twisted nematic (STN) mode, and thin film transistor (TFT) mode are known.
Liquid crystalline compounds used in these display modes must exhibit a liquid crystal phase in a wide temperature range with room temperature being its center, must be sufficiently stable under conditions in which display devices are used, and further must have characteristics sufficient to drive liquid crystal display devices. However, no liquid crystalline compounds which satisfy such requirements by a single compound have been found up to now. Accordingly, it is the actual circumstances that several or several tens of liquid crystalline compounds are mixed to prepare liquid crystal compositions having required characteristics. These liquid crystal compositions are required to be stable against moisture, light, heat, and air which usually exist under the conditions in which display devices are used; to be stable against electric field and electromagnetic radiation; and to be chemically stable against compounds to be mixed, in addition. Further, it is considered to be necessary that the liquid crystal compositions have an appropriate value of physical properties such as optical anisotropy (.DELTA.n) and dielectric anisotropy (.DELTA. .epsilon.), depending on the display mode and the shape of display devices.
Further, it is important that each component in liquid crystal compositions has good miscibility to one another. Particularly, more reduction of consumptive electric power and more lowering of threshold voltage which largely contributes to the high speed response necessary for applying to a wider size of liquid crystal displays (E. Jakeman et al., Phys. Lett., 39A. 69 (1972)) are desired; and it is also important for the high speed response that the liquid crystal compositions have a low viscosity.
In order to achieve these purposes, various compounds have heretofore been developed. Among them, compounds which contain silyl group in the molecule and are expressed by the following formula (a), formula (b), or formula (c) are published by Laid-open Japanese Patent Publication Nos. Hei 6-9653, Hei 7-2878, or Hei 7-2879, respectively. ##STR1##
However, these compounds have a trialkylsilyl group formed by substituting three alkyl groups for all three hydrogen atoms linked to silicon atom in silyl group and thus do not have one, particularly two or more unsubstituted hydrogen atoms.
As will be understood from the results of the determination of physical properties conducted by the present inventors, for example, for a compound which has propyldimethylsilyl group and is expressed by the following formula (d), compounds having a trialkylsilyl group have such problems that their viscosity is remarkably high, and besides that their miscibility with other components of liquid crystal compositions is not sufficient. ##STR2##
Results of the determination of physical properties of the compound of the formula (d) described above are shown below:
First, the phase transition temperature of a nematic phase-an isotropic phase (NI), and viscosity (.eta.) at 20.degree. C. were determined on a liquid crystal composition ZLI-1132 produced by Merck Co., Ltd. to be 72.6.degree. C. and 26.7 mPa.s, respectively.
Then, 15 % by weight of the compound expressed by the formula (d) was added to 85 % by weight of the liquid crystal composition, and NI and .eta. of the composition thus obtained were determined to be lower than 15.degree. C. and 39.7 mPa.s, respectively.
From the results, it can be understood that the viscosity of liquid crystal compositions prepared by using the compound expressed by the formula (d) remarkably increased and their NI lowered by more than 50.degree. C. Further, the compound expressed by the formula (d) is inadequate for practical purposes since the compound added to the mother liquid crystal composition had a portion having a different NI and was poor in miscibility.